1. Field of the Invention
The present invention, in general relates to cranks and, more particularly, to bicycle cranks that extend and retract during usage.
Pedal cranks that are powered by a human, such as the pedal crank of a bicycle, are well known. It is also known that the direction of the force applied to such a crank is fairly consistent. Normally an upright operator applies a downward force to the pedals. Depending upon the skill of the operator the magnitude of the force varies according to the position of the pedal in the stroke cycle. For example when the pedal is in the middle of the downward stroke, a skilled operator is applying maximum force to that pedal and when the pedal is in the upward stroke, the skilled operator would be applying a minimum amount of force to the pedal so as not to impede the direction of rotation of the crankshaft to which the pedals and cranks are attached.
Sometimes an accessory known as a toe-clip is attached to each of the pedals that allows a skilled user to apply a negative, or upward, force to the pedal during the upward stroke to gain efficiency. However, the bulk of the force that is normally applied to each of the pedals of a bicycle is downwardly directed.
Regardless as to whether toe-clips are used or not, all of the force that is applied to the pedal does not serve to advance rotation of the crankshaft, as is desired. A portion of the force acts radially upon the crankshaft in a direction that attempts to either extend or retract the crank. For example when the pedal is at the bottom of its stroke any force applied in an downward direction at that time would only attempt to extend the crank further away from the operator. It does not serve to advance rotation of the crankshaft.
Various solutions have been proposed. For example elliptical chain sprockets keep the pedal engaged in a position away from vertical for a greater proportion of the pedal cycle than do conventional circular sprockets by slowing the pedal speed while in the downward stroke.
This improvement is limited by acceptable fluctuations in pedal travel speed. To exaggerate the ellipse to further the benefit derived is unacceptable because it introduces too much of a variant in the speed of the pedal at various positions throughout the stroke.
Another improvement includes an extensible crank whereby the crank length is increased during the most useful part of the stroke to provide greater torque. However, there is no advancement of the rotation of the crankshaft that occurs inherent with the extension of the crank.
Other proposed solutions suffer from complexity of design and provide only limited benefit.
Accordingly there exists today a need for an extensible crank that translates extension of the crank into additional rotation of the crankshaft in the desired direction. Such an apparatus is clearly a useful and desirable device.
2. Description of Prior Art
Pedal operated cranks are, in general, known. For example, the following patents describe various types of these devices:
U.S. Pat. No. 4,319,271 to Chattin, that issued May 28, 1985;
U.S. Pat. No. 4,793,208 to Bregnard et al, that issued Dec. 27, 1988;
U.S. Pat. No. 4,807,491 to Stuckenbrok, that issued Feb. 28, 1989;
U.S. Pat. No. 5,067,370 to Lemmens, that issued Nov. 26, 1991;
U.S. Pat. No. 5,121,654 to Fasce, that issued Jun. 16, 1992; and
U.S. Pat. No. 5,419,572 to Stiller et al, that issued May 30, 1995.
While the structural arrangements of the above described devices, at first appearance, have similarities with the present invention, they differ in material respects. These differences, which will be described in more detail hereinafter, are essential for the effective use of the invention and which admit of the advantages that are not available with the prior devices.